The invention relates to adjustable steering column assemblies of the kind that use rows of inter-engaging teeth for the purpose of positively locking the steering wheel at various up/down (rake) and/or in/out (reach) positions.
Steering column assemblies are known which include a clamp mechanism that relies solely on friction when locked to prevent unwanted movement of the column assembly. The clamp mechanism can be unlocked and locked by movement of a locking lever or, sometimes, electrically using a motor or perhaps hydraulically or pneumatically. These columns can typically resist radial and/or axial forces of 600N applied at the steering wheel before slipping.
However, it is increasingly a requirement from car manufacturers that there should be no substantial movement of the steering column assembly when forces of up to 6000N are applied. The objective is to prevent unwanted movements of the steering wheel in a crash so that the deployment of the air bag is ideally controlled.
For this, friction alone is often insufficient, as massive clamp forces would be needed to achieve this. Some method of Positive Locking is needed. Typically this involves the use of inter-engaging teeth.
There is a well known problem with the type of “Positive-Locking” column which uses rows of inter-engaging teeth whereby, prior to operation of the locking lever by the driver, the selected steering wheel position has resulted in exact alignment of the tips of one row of teeth with those of the set of teeth with which they are supposed to engage. Inter-engaging of the teeth during the clamping operation is thereby blocked and, if excessive force is applied by the driver to the clamp lever to force it to complete its travel, permanent damage can occur to the tips of the teeth. At the very least, such a Tooth-on-Tooth occurrence will require the driver to consciously re-release the lever, find a slightly different wheel position and then re-engage the lever. Unless special means are provided to overcome this problem, then it is likely to occur during 10% to 20% of all adjustment operations. This is undesirable.